There is a need for inspection and sampling of persons, articles of clothing, buildings, furnishings, vehicles, baggage, cargo containers, dumpsters, packages, mail, and the like for contaminating residues (termed here more generally “trace analytes” or “target analytes”) that may indicate chemical hazards, explosives hazards, or illicit substances, while not limited thereto. Potential applications involve detection of trace materials, as both particles and vapors, associated with the presence of explosives on articles, on vehicles, or on persons, for example.
In particular, the recent increased threat to society from explosive devices and illicit drug traffic has lead to the development of sensitive systems for the detection of vapors and particles from explosives and drugs. Examples of prior art are described in U.S. Pat. Nos. 5,491,337, 6,708,572 and 6,642,513, among many. Equipment is available for hand search, desktop operation and walkthrough portal security applications, as has been reviewed by Hannum and Parmeter (Survey of commercially available explosives detection technologies and equipment, 1998, Sandia Natl Labs).
Current methods for surface sampling often involve contacting use of swabs or liquids, but methods for sampling by “sniffing” are preferred, where air in contact with a suspect package or person is drawn into a detection system directly. One prior art system, as described in U.S. Pat. No. 5,491,337, detects target analytes by ion mobility spectroscopy. Detectors of this class cannot tolerate humid air. Consequently, a dimethyl silicone membrane is used to reduce uptake of water vapor into the detector, while allowing the passage of at least a portion of the target analyte. Unfortunately the silicone membrane allows only a small part of the analyte vapors into the detector. Furthermore, the sensor does not capture particles, which can pass through the system without being detected.
McGown in U.S. Pat. No. 4,909,090 describes a hand-held vapor sampler, optionally with a shroud for enclosing a sampling space. The sampler uses low pressure puffs of hot air to vaporize illicit substances on surfaces and trap any vapors on a collector coil. The coil contains ribbon-like windings of metal which have a thin coating of material such as an organic polymer effective in absorbing organic molecules. However, particles are not sampled and would not be successfully aspirated under the conditions described, which relies on a 250 Watt lamp and a spring-actuated plunger for generating a puff of air. Improvements to the collector/desorber device are disclosed in U.S. Pat. No. 5,123,274 to Carroll.
U.S. Pat. No. 4,092,218 to Fine also discloses a method for the selective detection of explosives vapors, but does not show that it is capable of sampling particles. While it is tempting to limit collection to vapors alone, it is known that vapors of interest are frequently associated with or released from particles on heating, thus systems that collect only vapors may miss critical evidence.
Ishikawa in U.S. Pat. No. 7,275,453 discloses a cover enclosure in contact with a target surface, the enclosure with internally directed jet for operatively flushing and ejecting particles from the surface. Fine particles are collected by means of an inertial impactor and, at intervals, thermally flash gasified from the impactor for detection of chemical constituents by mass spectroscopy. A pair of impactors is used in parallel to increase sample throughput and to supply an uninterrupted gas feed to the mass spectrometer, however the sampling process remains a batch process. Also described is a heatable rotary trap, such as has long been known in the art.
To inspect mail or luggage, the sampling method of U.S. Pat. No. 6,887,710 involves first placing the article or articles in a box-like enclosure equipped with airlocks, directing a blast of air onto the exposed surfaces in order to dislodge particles associated with the articles, then sampling the gaseous contents of the box by drawing any resulting aerosol through a sampling port. However, the process is inherently slow because each article or person must be moved into the box or chamber and the box sealed before sampling, an obvious disadvantage when large numbers of articles or persons must be screened, or when the articles are larger than can be reasonably enclosed, such as a truck, shipping container, or the hallway surfaces of a building. Similar comments may be made regarding the teachings of U.S. Pat. No. 6,324,927 to Omath, where an enclosed shaker is used to dislodge particles.
Jenkins in U.S. Pat. No. 7,942,033 describes a method for sampling luggage by heating the luggage on a moving roller track and collecting vapors in the warm air plume rising from the hot luggage, optionally with vibration. In another embodiment, a stream of sampled air is impacted onto a porous filter, such that particles and “heavy molecules” are drawn into the filter by inertia, along with a flow of dry air, such that vapors pass through the filter for analysis, generally at elevated temperature. Unfortunately, continuous operation tends to clog the filter and reduces the filter temperature. Low filter temperature causes poor transmission of analyte, thus the apparatus is preferably operated in semi-continuous or batch mode. It is taught that vapor sampling is preferred over particle sampling because vapor is more closely associated with contraband.
Another approach, for sampling persons, is seen in U.S. Pat. No. 6,073,499 to Settles, aspects of which are also discussed in “Sniffers: fluid dynamic sampling for olfactory trace detection in nature and homeland security”, J Fluids Eng 127:189-218.
Various particle and vapor traps are disclosed in patents to Linker of Sandia Labs. The patents to Linker disclose methods to collect explosives particles for trace detectors that have some capability to collect vapor as well. U.S. Pat. No. 6,345,545, for example, discloses a two-stage preconcentrator that uses a metal or other electrically conducting screen to capture particles. Some vapors may also stick to the screen, however, the surface chosen for particle collection is not in general optimal for vapor collection. U.S. Pat. No. 6,523,393 discloses a hand-portable embodiment of the metal screen particle concentrator that makes use of a removable screen that is manually placed first in the high volume flow region and second in the detector region. Also to Linker are U.S. Pat. Nos. 7,299,711, 6,978,657, 6,604,406, 6,085,601, 5,854,431, and RE38,797.
Corrigan in U.S. Pat. Nos. 5,465,607 and 4,987,767, and Syage in U.S. Pat. No. 7,299,710, disclose systems for collection of particles and vapors. These systems operate in batch mode, resulting in interruption of sampling during processing Implementation has further proved difficult because particles have been found to poison commonly used vapor trap materials and means for efficiently separating particles from vapors are not recognized.
Also an unsolved problem is the elimination of vapor and gaseous interferences, such as water vapor, exogenous nitrous oxides, vehicle exhausts, sulfates, solvent vapors, and so forth, which are present in air and are known to interfere with or complicate a variety of analytical protocols. Thus direct collection of vapors is associated with both interferences and with loss of the particle signal, whereas combined collection can poison the vapor adsorbate. This dilemma is discussed in U.S. patent Ser. Nos. 13/078,997 and 13/078,999, which are co-assigned and are incorporated herein in entirety for all purposes by reference.
Thus, there is a need in the arts associated with aerosol surveillance and analysis for an apparatus and methods that overcome the above disadvantages and limitations. Of potential interest in high-throughput screening luggage, vehicles and persons is a system capable of continuous sampling at high flow rate, typically at a flow rate greater than 100 L/min, where particles are separated from interfering gases, and then the volatile constituents associated with the particles are vaporized and sampled at low flow rate, typically at or less than 1 L/min, most preferably at 50 to 150 milliliters per minute, or less. A preferred system requires little maintenance and operates with a relative absence of moving parts such that the gas phase is the vehicle for both selectively separating the particles from the bulk flow and for conveying vapors selectively stripped from the particles to an analytical unit.